Method and apparatus for synchronizing measurements taken by multiple metrology devices

ABSTRACT

Apparatus includes at least two devices that communicate with each other, wherein a first one of the at least two devices having an IEEE 1588 precision time protocol interface, the interface including one or more components configured for communications in both a wired manner and a wireless manner with a second one of the at least two devices. The second one of the at least two devices having an IEEE 1588 precision time protocol interface, the interface including one or more components configured for communications in both a wired manner and a wireless manner with the first one of the at least two devices. Wherein one of the at least two devices includes a master clock and the other one of the at least two devices includes a slave clock, wherein the master clock communicates a time to the slave clock and the slave clock is responsive to the communicated time from the master clock to adjust a time of the slave clock if necessary to substantially correspond to the time of the master clock, thereby time synchronizing the at least two devices together.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of provisional applicationNo. 61/293,838 filed Jan. 11, 2010, and provisional application61/296,555 filed Jan. 20, 2010, the contents of each of which are herebyincorporated by reference in their entirety.

BACKGROUND

The invention relates generally to the time synchronization ofmeasurements taken by multiple measurement devices, and moreparticularly to the time synchronization of measurements taken bymultiple metrology devices such as articulated arm coordinate measuringmachines, laser trackers, laser scanners, and other types of partsmeasurement precision metrology devices.

For example, one of these metrology devices may belong to a class ofinstruments that measure the coordinates of each of a number of pointson an object or part to be measured by probing the point with anarticulated mechanical structure. The probing may be performed with acontacting mechanical probe tip and/or with a non-contacting scanningdevice (e.g., a laser line probe (LLP)). The position of the probe tipor scanning device relative to the base of the structure (i.e., in acertain coordinate frame of reference) is determined by the readings ofangular encoders located at the connection points (e.g., bearingcartridges) of the articulating arm segments. This type of device,whether it uses a mechanical probe tip or a scanner, is referred to asan articulated arm coordinate measuring machine.

Portable articulated arm coordinate measuring machines (AACMMs) havefound widespread use in the manufacturing or production of parts wherethere is a need to rapidly and accurately verify the dimensions of thepart during various stages of the manufacturing or production (e.g.,machining) of the part. Portable AACMMs represent a vast improvementover known stationary or fixed, cost-intensive and relatively difficultto use measurement installations, particularly in the amount of time ittakes to perform dimensional measurements of relatively complex parts.Typically, a user of a portable AACMM simply guides a probe along thesurface of the part or object to be measured. The measurement data arethen recorded and provided to the user. In some cases, the data areprovided to the user in visual form, for example, three-dimensional(3-D) form on a computer screen. In other cases, the data are providedto the user in numeric form, for example when measuring the diameter ofa hole, the text “Diameter=1.0034” is displayed on a computer screen.

An example of a prior art portable articulated arm CMM is disclosed incommonly assigned U.S. Pat. No. 5,402,582 ('582), which is incorporatedherein by reference in its entirety. The '582 patent discloses a 3-Dmeasuring system comprised of a manually-operated articulated arm CMMhaving a support base on one end and a measurement probe at the otherend. Commonly assigned U.S. Pat. No. 5,611,147 ('147), which isincorporated herein by reference in its entirety, discloses a similararticulated arm CMM. In the '147 patent, the articulated arm CMMincludes a number of features including an additional rotational axis atthe probe end, thereby providing for an arm with either a two-two-two ora two-two-three axis configuration (the latter case being a seven axisarm).

Another type of these devices may be an instrument, referred to as alaser tracker, which measures the coordinates of a point by sending alaser beam to a retroreflector target that is in contact with the point.The laser tracker determines the coordinates of the point by measuringthe distance and the two angles to the retroreflector. The distance ismeasured with a distance-measuring device such as an absolute distancemeter or an interferometer. The angles are measured with anangle-measuring device such as an angular encoder. A gimbaledbeam-steering mechanism within the instrument directs the laser beam tothe point of interest. Exemplary systems belonging to this class ofinstruments are described by U.S. Pat. No. 4,790,651 to Brown et al. andU.S. Pat. No. 4,714,339 to Lau et al.

A laser tracker can measure relatively large parts (i.e., parts largerthan what a portable AACMM can measure without relocation of the AACMM)and in a shorter amount of time as compared to a portable AACMM,especially if the portable AACMM must be relocated to completely measurethe part. Modern laser trackers can measure relatively large parts with3-D single-point accuracy to 0.001 inches. Such a laser trackertypically uses its laser to measure the 3-D coordinates or a point at arange of up to 230 ft (70 m) by following the movement of aretroreflector such as a spherically mounted retroreflector (SMR) andreport on the measured position in real time. Some modern laser trackerscan provide real time updates of SMR positions.

There are several types of laser scanners, but all types project lightonto objects to be tested or measured. Most surfaces of these objectsare diffusely scattering, and measurements are ordinarily made withoutthe assistance of a cooperative target such as a retroreflector. Somescanners, such as the one that attaches to the end of the AACMMdescribed above (e.g., an LLP), are moved by hand and the laser light isdirected over a surface of the object. The contacting probe of theportable AACMM and the non-contacting laser line probe can digitize datainterchangeably without having to remove either component from theAACMM. A laser line probe provides multiple sampling points to be takensimultaneously along an object's surface, which is illuminated by thelaser scan line. Users can accurately measure prismatic features withthe AACMM contacting probe, then laser scan sections requiring largeramounts of data (detailed feature extraction) at more than 19,000 pointsper-second—without adding or removing attachments. An exemplarynon-contacting scanner of this type is described in commonly assignedU.S. Pat. No. 6,965,843 to Raab et al., which is incorporated herein byreference in its entirety.

Other scanners are stationary and illuminate an entire area of interest.An exemplary scanner of this type is described in U.S. Pat. No.7,599,071 to Dillon et al. A third type of laser scanner sends a laserbeam over a scan pattern that covers a large volume. A laser scanner ofthis type may sometimes measure the 3-D coordinates of a relativelylarge volume within a few minutes. An exemplary laser scanner of thistype is described in commonly assigned U.S. Pat. No. 7,430,068 to Beckeret al., which is incorporated by reference herein in its entirety.

There are many situations in which multiple metrology devices, eitherall of the same type or of different types, being connected, arranged orcombined in a distributed networked system, may benefit from beingsynchronized in time to one another. In general, as distributed networktechnologies increase in number and complexity, these systemtechnologies applied to measurement and control become more complex asthe number of nodes (i.e., metrology devices) in the system increases.It has become common to utilize local real time clocks in the variousdevices to achieve relatively accurate system-wide time. However, eachof these individual clocks tends to drift apart from one another (i.e.,lose their system-wide synchronization) due to, for example, initialfrequency offsets, instabilities in the clock oscillators andenvironmental conditions such as temperature, aging, vibration,mechanical stresses, etc. As such, the measurements taken by thesevarious devices and any resulting control imparted to the overall systemsuffer from the time inaccuracies of the individual clocks. Thus, sometype of clock time synchronization correction or adjustment is needed toproperly synchronize the individual clocks to thereby maintain anaccurate and common measure of system-wide time.

The following outlines five examples in which precise timesynchronization benefits system combinations of precision metrologydevices.

Tracker and Arm: The portable AACMM is capable of being moved orpositioned into a variety of different orientations. Because of this,the portable AACMM is able to measure “hidden” points; that is, pointsthat are hidden from a line-of-sight view of a measuring device such asa laser tracker. On the other hand, the laser tracker can measure over amuch larger volume than the AACMM. It is possible to use a laser trackerto relocate a portable AACMM by attaching a retroreflector to the armportion of the AACMM. In this way the best features of each device areretained, while enabling the combined system to accurately measurehidden points over a relatively large volume.

An exemplary method for physically relocating the portable AACMM in thisway is described in commonly assigned U.S. Pat. No. 7,804,602 to Raab,which is incorporated by reference herein in its entirety. To obtain thebest possible relocation, it is important that the portable AACMM andthe laser tracker be accurately synchronized in time to one another sothat the corresponding measurements made by each device (e.g., of theposition of the retroreflector) are accurately synchronized during therelocation process.

Simultaneous multilateration: By using the highly accurate distancemeters (e.g., interferometers or absolute distance meters) of a multipleof laser trackers to simultaneously measure a single, wide-angleretroreflector target, the 3-D coordinates of the target can be measuredto a relatively high degree of accuracy. It is desirable to make such asimultaneous multilateration measurement on a moving target. Thisenables the target to be moved over surfaces of objects of interest,thereby producing a map of the surface contours. To retain therelatively high accuracy obtained with this method, the multipletrackers must be accurately synchronized in time.

Wireless synchronization of scanner on end of AACMM: Advances intechnology have caused a rapid increase in the rate of data collected byscanners attached to AACMMs, typically at the probe end. This increasingdata rate is making it relatively more difficult to successfullytransmit data from the scanner located at one end (e.g., the probe end)of the AACMM to the computing elements typically located within the baseat the other end of the AACMM. A way around this problem is to send datawirelessly from the scanner to the computer or computing elements. Forthis wireless approach to be practical, it is important that the partmeasurement data from the scanner be accurately synchronized with probepositional data from the angular encoders within the AACMM.

Compensation and calibration of laser scanner: It is often necessary tocompensate or calibrate a laser scanner of the type described abovewhich sends a laser beam over a scan pattern that covers a relativelylarge volume. This type of scanner may sometimes measure 3-D coordinatesof such a large volume within a few minutes. One way to do this is tocompare readings of the scanner to those of a more accurate instrumentsuch as a laser tracker. This may be done by placing a target on acarriage mounted on a motorized rail. The scanner is placed on one endof the rail and points its laser beam to a suitable diffuse target. Alaser tracker is placed on the other end of the rail and points itslaser beam to a suitable retroreflector target. The diffuse andretroreflector targets are placed back to back and face in oppositedirections. To speed up the collection of data, it is desirable tocollect data from both the scanner and the tracker while the targetassembly is moving. This is only possible if the scanner and tracker areaccurately synchronized.

Compensation of AACMM: It is possible to find compensation parameters ofan AACMM by moving the arm segments of the articulated arm portion ofthe AACMM into a variety of different positions while a laser trackerfollows a retroreflector target attached to the arm. Compensationparameters are found by comparing the 3-D readings from the lasertracker to readings of encoders in the AACMM. For this method to bepractical, the arm segments must be moved into a wide variety ofpositions while the tracker records retroreflector position data at arelatively high rate. This method is accurate only if the laser trackerand the AACMM are accurately synchronized.

In all of the above example cases, what is needed is a way tosynchronize the measurements taken between multiple metrology devices—insome cases, different types of multiple metrology devices, in othercases, similar types of multiple metrology devices.

Time synchronization is necessary and is already available within aportable AACMM. For example, the plurality of angular encoders used inan AACMM are sampled simultaneously. Similarly, the laser interferometerand absolute distance meter in a laser tracker are sampled in such a waythat measurement results for each correspond to the same instants intime. Such synchronization is relatively easy to do in a single piece ofequipment as a common clock is available. However, this situationchanges when multiple portable AACMMs and/or other measurement devicesare used in a coordinated measurement manner. In this case, typically amaster clock is provided and a means by which the devices aresynchronized to the master clock is also provided.

Previous methods of time synchronization of precision measurementdevices have an inherent problem of a relatively large and unacceptablesynchronization error. To demonstrate the problems created bysynchronization error, consider the case where the time error is onemillisecond in synchronization. If one of the measurement devices in asystem of multiple measurement devices is a laser tracker measuring aretroreflector moving at one meter per second, the resultant error inmeasured distance is one millimeter. However, if an accuracy of tenmicrometers is needed, the error is one hundred times larger thanacceptable. Also, for that situation (as in a prior art case) withoutany synchronization of the clocks driving the individual metrologydevices, the fact that a measurement has to wait for the respective“sync” signals to occur introduces additional synchronization delays.For example, an additional millisecond may be added to the totalpropagation delay.

For example, one prior art system used one laser tracker as a mastertracker and a second laser tracker as a slave tracker. The master sentout a strobe signal that was received by the slave. The strobe was sentthrough cable drivers and was intercepted by receivers in the tracker.This signal went to a microprocessor that implemented a measurement taskon that slave tracker as well as the master tracker. This process causedabout one millisecond of delay. Cable propagation andcapacitance/resistance of the circuit components accounted for much ofthis delay.

The IEEE 1588 precision time protocol (PTP), which implements a masterclock in a phase locked loop manner, has also been proposed for use inrobotics, for example as discussed in U.S. Published Patent ApplicationNo. 2006/0287769 to Yanagita. However, Yanagita at paragraph [0007] doesnot in fact recommend use of IEEE 1588. Instead, Yanagita states thatIEEE 1588 requires specialized and expensive hardware, and insteadproposes a programmed software solution using a master and slave tickcount for synchronization of two or more robotic arms. In general,robotic devices used in manufacturing seldom have a need to achieve the500 ns (nanosecond) time synchronization accuracy required by metrologyinstruments for applications of the types described above. Thus, thefields of robotics and metrology do not have as much in common as acursory examination might suggest.

What is needed is apparatus and a method for relatively precise timesynchronization (e.g., 500 nanoseconds maximum) of the real time clockswithin a multiple of similar or different metrology devices using theIEEE 1588 precision time protocol (PTP), to thereby preciselysynchronize the measurements taken by the metrology devices to adesired, relatively high level of accuracy.

SUMMARY OF THE INVENTION

According to embodiments of a first aspect of the of the presentinvention, apparatus includes at least two devices that communicate witheach other, wherein a first one of the at least two devices having anIEEE 1588 precision time protocol interface, the interface including oneor more components configured for communications in both a wired mannerand a wireless manner with a second one of the at least two devices. Thesecond one of the at least two devices having an IEEE 1588 precisiontime protocol interface, the interface including one or more componentsconfigured for communications in both a wired manner and a wirelessmanner with the first one of the at least two devices. Wherein one ofthe at least two devices includes a master clock and the other one ofthe at least two devices includes a slave clock, wherein the masterclock communicates a time to the slave clock and the slave clock isresponsive to the communicated time from the master clock to adjust atime of the slave clock if necessary to substantially correspond to thetime of the master clock, thereby time synchronizing the at least twodevices together.

According to embodiments of a second aspect of the present invention, amethod for communicating between at least two devices includes the stepsof: providing a first one of the at least two devices with an IEEE 1588precision time protocol interface configured for communications in botha wired manner and a wireless manner with a second one of the at leasttwo devices; providing the second one of the at least two devices withan IEEE 1588 precision time protocol interface configured forcommunications in both a wired manner and a wireless manner with thefirst one of the at least two devices; and wherein one of the at leasttwo devices includes a master clock and the other one of the at leasttwo devices includes a slave clock, the master clock communicating atime to the slave clock and the slave clock is responsive to thecommunicated time from the master clock for adjusting a time of theslave clock if necessary to substantially correspond to the time of themaster clock, thereby time synchronizing the at least two devicestogether.

According to embodiments of a third aspect of the present invention, acomputer program product for implementing communications between atleast two devices includes a storage medium having computer readableprogram code embodied thereon, which when executed by a computer causesthe computer to implement a method, the method including the steps of:providing a first one of the at least two devices with an IEEE 1588precision time protocol interface configured for communications in botha wired manner and a wireless manner with a second one of the at leasttwo devices; providing the second one of the at least two devices withan IEEE 1588 precision time protocol interface configured forcommunications in both a wired manner and a wireless manner with thefirst one of the at least two devices; and wherein one of the at leasttwo devices includes a master clock and the other one of the at leasttwo devices includes a slave clock, the master clock communicating atime to the slave clock and the slave clock is responsive to thecommunicated time from the master clock for adjusting a time of theslave clock if necessary to substantially correspond to the time of themaster clock, thereby time synchronizing the at least two devicestogether.

The above discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, exemplary embodiments are shown whichshould not be construed to be limiting regarding the entire scope of thedisclosure, and wherein the elements are numbered alike in severalFIGURES:

FIG. 1, including FIGS. 1A and 1B, are perspective views of a portablearticulated arm coordinate measuring machine (AACMM) having embodimentsof various aspects of the present invention therewithin;

FIG. 2, including FIGS. 2A-2D taken together, is a block diagram ofelectronics utilized as part of the AACMM of FIG. 1 in accordance withan embodiment;

FIG. 3, including FIGS. 3A and 3B taken together, is a block diagramdescribing detailed features of the electronic data processing system ofFIG. 2 in accordance with an embodiment;

FIG. 4 is a perspective view of the portable AACMM of FIG. 1 used inconjunction with a laser tracker in embodiments of the presentinvention;

FIG. 5 is an exploded, perspective view of a retroreflector clampassembly used with the portable AACMM of FIG. 1 in the embodiments ofthe present invention of FIG. 4;

FIG. 6 is a perspective view of the portable AACMM of FIG. 1 relocatedto a second position through the use of the laser tracker in theembodiments of the present invention of FIG. 4;

FIG. 7 is a perspective view of a mounted sphere assembly utilized withthe portable AACMM of FIG. 1 in the embodiments of the present inventionof FIG. 4;

FIG. 8, including FIGS. 8A and 8B, are perspective views of the portableAACMM of FIG. 1 in contact with the mounted sphere assembly of FIG. 5 inembodiments of the present invention of FIG. 4;

FIG. 9, including FIGS. 9A and 9B, are perspective views similar tothose of FIG. 8 of the portable AACMM of FIG. 1 in contact with themounted sphere assembly of FIG. 5 in embodiments of the presentinvention of FIG. 4 with reference made to a coordinate frame ofreference;

FIG. 10 is a diagram showing a grandmaster clock being time synchronizedto one or more slave clocks using the IEEE 1588 precision time protocol;

FIGS. 11 and 12 are diagrams illustrating the IEEE 1588 offset and delaymeasurement as part of the time synchronization procedure between thegrandmaster clock and the one or more slave clocks of FIG. 10;

FIG. 13 is a block diagram of a variable hardware clock used within aslave clock and having a phase accumulator as in the prior art;

FIG. 14 is a block diagram of IEEE 1588 PTP clock control using a phaseaccumulator as in the prior art;

FIG. 15 is a block diagram showing a plurality of metrology devicestogether with other devices connected by wired and wireless connectionsto a wireless router and on to a network;

FIG. 16, including FIGS. 16A-16C taken together, is a block diagramillustrating a simplified view of a wireless router having IEEE 1588 PTPmaster, boundary or optional transparent clock support;

FIG. 17 is a block diagram illustrating the hierarchy of clocks utilizedby the IEEE 1588 PTP in an exemplary embodiment of the present inventionwith various wired nodes in a LAN domain and various wireless stationsin a WLAN domain connected by wired and wireless connections to acentral router;

FIG. 18 is a block diagram illustrating the functions performed within aslave device implementing the IEEE 1588 PTP;

FIG. 19 is a more detailed block diagram of a hardware timestamp withinan IEEE 1588 PTP PHY device;

FIG. 20, including FIGS. 20A and 20B taken together, is a functionaldiagram of timing control and external signal generation within an IEEE1588 PTP PHY device;

FIG. 21 is a functional block diagram of a synchronous Ethernetconfiguration of a plurality of nodes;

FIG. 22 is a functional block diagram of the functions within an IEEE1588 PTP PHY device for supporting the synchronous Ethernetconfiguration of FIG. 21; and

FIG. 23 is a functional block diagram of a wireless station thatincludes a wireless LAN module used as a means of bridging a wirelessnetwork with a wired Ethernet network.

DETAILED DESCRIPTION

FIGS. 1A and 1B illustrate, in perspective, a portable articulated armcoordinate measuring machine (AACMM) 100 according to variousembodiments of the present invention, an articulated arm being one typeof coordinate measuring machine. As shown in FIGS. 1A and 1B, theexemplary AACMM 100 may comprise a six or seven axis articulatedmeasurement device having a measurement probe housing 102 coupled to anarm portion 104 of the AACMM 100 at one end. The arm portion 104comprises a first arm segment 106 coupled to a second arm segment 108 bya first grouping of bearing cartridges 110 (e.g., two bearingcartridges). A second grouping of bearing cartridges 112 (e.g., twobearing cartridges) couples the second arm segment 108 to themeasurement probe housing 102. A third grouping of bearing cartridges114 (e.g., three bearing cartridges) couples the first arm segment 106to a base 116 located at the other end of the arm portion 104 of theAACMM 100. Each grouping of bearing cartridges 110, 112, 114 providesfor multiple axes of articulated movement. Also, the measurement probehousing 102 may comprise the shaft of the seventh axis portion of theAACMM 100 (e.g., a cartridge containing an encoder system thatdetermines movement of the measurement device, for example a probe 118,in the seventh axis of the AACMM 100). In use of the AACMM 100, the base116 is typically affixed to a work surface.

Each bearing cartridge within each bearing cartridge grouping 110, 112,114 typically contains an encoder system (e.g., an optical angularencoder system). The encoder system (i.e., transducer) provides anindication of the position of the respective arm segments 106, 108 andcorresponding bearing cartridge groupings 110, 112, 114 that alltogether provide an indication of the position of the probe 118 withrespect to the base 116 (and, thus, the position of the object beingmeasured by the AACMM 100 in a certain frame of reference—for example alocal or global frame of reference). The arm segments 106, 108 may bemade from a suitably rigid material such as but not limited to a carboncomposite material for example. A portable AACMM 100 with six or sevenaxes of articulated movement (i.e., degrees of freedom) providesadvantages in allowing the operator to position the probe 118 in adesired location within a 360° area about the base 116 while providingan arm portion 104 that may be easily handled by the operator. However,it should be appreciated that the illustration of an arm portion 104having two arm segments 106, 108 is for exemplary purposes, and theclaimed invention should not be so limited. An AACMM 100 may have anynumber of arm segments coupled together by bearing cartridges (and,thus, more or less than six or seven axes of articulated movement ordegrees of freedom).

The probe 118 is detachably mounted to the measurement probe housing102, which is connected to bearing cartridge grouping 112. A handle 126is removable with respect to the measurement probe housing 102 by wayof, for example, a quick-connect interface. The handle 126 may bereplaced with another device (e.g., a laser line probe, a bar codereader), thereby providing advantages in allowing the operator to usedifferent measurement devices with the same AACMM 100. In exemplaryembodiments, the probe housing 102 houses a removable probe 118, whichis a contacting measurement device and may have different tips 118 thatphysically contact the object to be measured, including, but not limitedto: ball, touch-sensitive, curved and extension type probes. In otherembodiments, the measurement is performed, for example, by anon-contacting device such as a laser line probe (LLP). In anembodiment, the handle 126 is replaced with the LLP using thequick-connect interface. Other types of measurement devices may replacethe removable handle 126 to provide additional functionality. Examplesof such measurement devices include, but are not limited to, one or moreillumination lights, a temperature sensor, a thermal scanner, a bar codescanner, a projector, a paint sprayer, a camera, or the like, forexample.

As shown in FIGS. 1A and 1B, the AACMM 100 includes the removable handle126 that provides advantages in allowing accessories or functionality tobe changed without removing the measurement probe housing 102 from thebearing cartridge grouping 112. As discussed in more detail below withrespect to FIG. 2, the removable handle 126 may also include anelectrical connector that allows electrical power and data to beexchanged with the handle 126 and the corresponding electronics locatedin the probe end.

In various embodiments, each grouping of bearing cartridges 110, 112,114 allows the arm portion 104 of the AACMM 100 to move about multipleaxes of rotation. As mentioned, each bearing cartridge grouping 110,112, 114 includes corresponding encoder systems, such as optical angularencoders for example, that are each arranged coaxially with thecorresponding axis of rotation of, e.g., the arm segments 106, 108. Theoptical encoder system detects rotational (swivel) or transverse (hinge)movement of, e.g., each one of the arm segments 106, 108 about thecorresponding axis and transmits a signal to an electronic dataprocessing system within the AACMM 100 as described in more detailherein below. Each individual raw encoder count is sent separately tothe electronic data processing system as a signal where it is furtherprocessed into measurement data. No position calculator separate fromthe AACMM 100 itself (e.g., a serial box) is required, as disclosed incommonly assigned U.S. Pat. No. 5,402,582 ('582).

The base 116 may include an attachment device or mounting device 120.The mounting device 120 allows the AACMM 100 to be removably mounted toa desired location, such as an inspection table, a machining center, awall or the floor for example. In one embodiment, the base 116 includesa handle portion 122 that provides a convenient location for theoperator to hold the base 116 as the AACMM 100 is being moved. In oneembodiment, the base 116 further includes a movable cover portion 124that folds down to reveal a user interface, such as a display screen.

In accordance with an embodiment, the base 116 of the portable AACMM 100contains or houses an electronic data processing system that includestwo primary components: a base processing system that processes the datafrom the various encoder systems within the AACMM 100 as well as datarepresenting other arm parameters to support three-dimensional (3-D)positional calculations; and a user interface processing system thatincludes an on-board operating system, a touch screen display, andresident application software that allows for relatively completemetrology functions to be implemented within the AACMM 100 without theneed for connection to an external computer.

The electronic data processing system in the base 116 may communicatewith the encoder systems, sensors, and other peripheral hardware locatedaway from the base 116 (e.g., a LLP that can be mounted to the removablehandle 126 on the AACMM 100). The electronics that support theseperipheral hardware devices or features may be located in each of thebearing cartridge groupings 110, 112, 114 located within the portableAACMM 100.

FIG. 2 is a block diagram of electronics utilized in an AACMM 100 inaccordance with an embodiment. The embodiment shown in FIG. 2 includesan electronic data processing system 210 including a base processorboard 204 for implementing the base processing system, a user interfaceboard 202, a base power board 206 for providing power, a Bluetoothmodule 232, and a base tilt board 208. The user interface board 202includes a computer processor for executing application software toperform user interface, display, and other functions described herein.

As shown in FIG. 2, the electronic data processing system 210 is incommunication with the aforementioned plurality of encoder systems viaone or more arm buses 218. In the embodiment depicted in FIG. 2, eachencoder system generates encoder data and includes: an encoder arm businterface 214, an encoder digital signal processor (DSP) 216, an encoderread head interface 234, and a temperature sensor 212. Other devices,such as strain sensors, may be attached to the arm bus 218.

Also shown in FIG. 2 are probe end electronics 230 that are incommunication with the arm bus 218. The probe end electronics 230include a probe end DSP 228, a temperature sensor 212, a handle/LLPinterface bus 240 that connects with the handle 126 or the LLP 242 viathe quick-connect interface in an embodiment, and a probe interface 226.The quick-connect interface allows access by the handle 126 to the databus, control lines, and power bus used by the LLP 242 and otheraccessories. In an embodiment, the probe end electronics 230 are locatedin the measurement probe housing 102 on the AACMM 100. In an embodiment,the handle 126 may be removed from the quick-connect interface andmeasurement may be performed by the laser line probe (LLP) 242communicating with the probe end electronics 230 of the AACMM 100 viathe handle/LLP interface bus 240. In an embodiment, the electronic dataprocessing system 210 is located in the base 116 of the AACMM 100, theprobe end electronics 230 are located in the measurement probe housing102 of the AACMM 100, and the encoder systems are located in the bearingcartridge groupings 110, 112, 114. The probe interface 226 may connectwith the probe end DSP 228 by any suitable communications protocol,including commercially-available products from Maxim IntegratedProducts, Inc. that embody the 1-wire® communications protocol 236.

FIG. 3 is a block diagram describing detailed features of the electronicdata processing system 210 of the AACMM 100 in accordance with anembodiment. In an embodiment, the electronic data processing system 210is located in the base 116 of the AACMM 100 and includes the baseprocessor board 204, the user interface board 202, a base power board206, a Bluetooth module 232, and a base tilt module 208.

In an embodiment shown in FIG. 3, the base processor board 204 includesthe various functional blocks illustrated therein. For example, a baseprocessor function 302 is utilized to support the collection ofmeasurement data from the AACMM 100 and receives raw arm data (e.g.,encoder system data) via the arm bus 218 and a bus control modulefunction 308. The memory function 304 stores programs and static armconfiguration data. The base processor board 204 also includes anexternal hardware option port function 310 for communicating with anyexternal hardware devices or accessories such as an LLP 242. A real timeclock (RTC) and log 306, a battery pack interface (IF) 316, and adiagnostic port 318 are also included in the functionality in anembodiment of the base processor board 204 depicted in FIG. 3.

The base processor board 204 also manages all the wired and wirelessdata communication with external (host computer) and internal (displayprocessor 202) devices. The base processor board 204 has the capabilityof communicating with an Ethernet network via an Ethernet function 320(e.g., using a clock synchronization standard such as Institute ofElectrical and Electronics Engineers (IEEE) 1588), with a wireless localarea network (WLAN) via a LAN function 322, and with Bluetooth module232 via a parallel to serial communications (PSC) function 314. The baseprocessor board 204 also includes a connection to a universal serial bus(USB) device 312.

The base processor board 204 transmits and collects raw measurement data(e.g., encoder system counts, temperature readings) for processing intomeasurement data without the need for any preprocessing, such asdisclosed in the serial box of the aforementioned '582 patent. The baseprocessor 204 sends the processed data to the display processor 328 onthe user interface board 202 via an RS485 interface (IF) 326. In anembodiment, the base processor 204 also sends the raw measurement datato an external computer.

Turning now to the user interface board 202 in FIG. 3, the angle andpositional data received by the base processor is utilized byapplications executing on the display processor 328 to provide anautonomous metrology system within the AACMM 100. Applications may beexecuted on the display processor 328 to support functions such as, butnot limited to: measurement of features, guidance and training graphics,remote diagnostics, temperature corrections, control of variousoperational features, connection to various networks, and display ofmeasured objects. Along with the display processor 328 and a liquidcrystal display (LCD) 338 (e.g., a touch screen LCD) user interface, theuser interface board 202 includes several interface options including asecure digital (SD) card interface 330, a memory 332, a USB Hostinterface 334, a diagnostic port 336, a camera port 340, an audio/videointerface 342, a dial-up/cell modem 344 and a global positioning system(GPS) port 346.

The electronic data processing system 210 shown in FIG. 3 also includesa base power board 206 with an environmental recorder 362 for recordingenvironmental data. The base power board 206 also provides power to theelectronic data processing system 210 using an AC/DC converter 358 and abattery charger control 360. The base power board 206 communicates withthe base processor board 204 using inter-integrated circuit (I2C) serialsingle ended bus 354 as well as via a DMA serial peripheral interface(DSPI) 356. The base power board 206 is connected to a tilt sensor andradio frequency identification (RFID) module 208 via an input/output(I/O) expansion function 364 implemented in the base power board 206.

Though shown as separate components, in other embodiments all or asubset of the components may be physically located in differentlocations and/or functions combined in different manners than that shownin FIG. 3. For example, in one embodiment, the base processor board 204and the user interface board 202 are combined into one physical board.

A plurality of different types of metrology devices, or a plurality ofsimilar metrology devices, which are arranged, linked, connected,associated, configured, or set up together, whether or not physicallyattached in some manner, for example, as a system, may utilizeembodiments of the time synchronization apparatus and method accordingto the present invention disclosed herein in order to accuratelysynchronize the various measurements taken by the multiple metrologydevices within the system. In some embodiments, even non-metrologydevices, such as for example a robot or a camera, may be utilized inembodiments of the present invention.

An exemplary embodiment of the present invention includes a system thatcomprises an articulated arm coordinate measuring machine, such as theportable AACMM 100 described herein above with respect to FIGS. 1-3, anda laser tracker configured together with respect to each other asdescribed in detail herein after. The portable AACMM 100 and the lasertracker are time synchronized using the IEEE 1588 precision timeprotocol (PTP) standard, for example, for taking measurements of anobject. More particularly, the exemplary embodiment described hereinafter relates to a portable AACMM 100 and a laser tracker used togetherto relocate the portable AACMM 100 to different physical locations sothat the portable AACMM 100 can completely measure a relatively largepart or object (e.g., an automobile), where such complete measurement isnot possible if the portable AACMM 100 is not relocated to additionalphysical locations.

As described herein above, portable AACMMs 100 are used to obtain threedimensional position data with respect to an object, such as a partundergoing measurement. The portable AACMM 100 converts the physicalobject to a set of data representing the physical object. Additionaldetails regarding portable AACMMs are found in the aforementionedcommonly assigned U.S. Pat. No. 5,402,582 ('582) to Raab, which isincorporated herein by reference in its entirety.

A modern portable AACMM 100 is capable of measuring in a volume definedas being within a range extending up to the maximum reach of the armportion 104 of the AACMM 100. This maximum arm reach may, for example,typically be six or twelve feet, depending primarily on the length ofthe arm segments 106, 108. Measuring outside of or beyond this volumewhile still maintaining an original or global coordinate system or frameof reference requires physically relocating the portable AACMM 100 toone or more other different physical locations and performing acoordinate system transfer procedure at each location, for example theprocedure referred to as “Leap Frog.” In the Leap Frog procedure,features in the first measurement volume (i.e., with the portable AACMM100 in its initial physical location) are used to reset the coordinatesystem of the portable AACMM 100 after moving the portable AACMM 100 toa second, new or different physical location to continue the partmeasurement process (for example, when measuring an automobile orsimilarly large object). The portable AACMM 100 may also be relocated toa third, a fourth or additional different physical locations asnecessary to successfully complete the part measurement process. The netresult of the Leap Frog procedure is a stacking of any error since theportable AACMM 100 is consecutively used to re-establish its coordinatesystem. A more detailed description of the Leap Frog procedure is foundin U.S. Pat. No. 4,430,796, which is incorporated herein in its entiretyby reference.

An example of a relatively large-scale coordinate probing measurementsystem 400 is shown in FIG. 4, which is similar to the measurementsystem disclosed in the aforementioned commonly assigned U.S. Pat. No.7,804,602, which is hereby incorporated by reference in its entirety.The probing system 400 of FIG. 4 comprises a portable AACMM 100 similarto that described herein above with respect to FIGS. 1-3 and shown insimplified form in FIG. 4. The system also comprises a retroreflectorclamp assembly 410 located on the portable AACMM 100 at or near theprobe end 420 of the AACMM 400, and a laser tracker 430. Otherorientations, arrangements, set-ups, and variations of multiplemeasurement devices of the same type or different types are possible andcontemplated by embodiments of the present invention, depending, forexample, upon the specific application in the field. Thus, the exemplarysystem 400 of FIG. 4 should not be considered to be limiting.

FIG. 5 shows an exploded view of retroreflector clamp assembly 410,which comprises spherically mounted retroreflector (SMR) 500, kinematicnest 510, and clamp 520. SMR 500 comprises cube-corner retroreflector530 embedded within partial sphere 540. Cube-corner retroreflector 530comprises three flat mirror segments (M1, M2, M3), which are joinedtogether in such a way that each glass segment makes a ninety-degreeangle with respect to the other two glass segments. The point of commonintersection of the three glass segments is called the apex “A” of SMR500. The apex “A” is located at the spherical center of partial sphere540.

Kinematic nest 510 attaches to the top of clamp 520, which in turn locksonto the final link or arm segment 550 of the portable AACMM 100 at ornear the probe end 420. Thus, the clamp 520 allows the retroreflectorclamp assembly 410 to be placed onto the portable AACMM 100 at a desiredlocation.

Kinematic nest 510 has three point-like contacts (not shown) onto whichthe spherical surface of SMR 500 rests. These point-like contacts ensurethat the center of SMR 500 remains at the same point in space as SMR 500is rotated. Kinematic nest 510 preferably contains a magnet in its baseto ensure that SMR 500 is kept in constant contact with the threepoint-like contacts.

As shown in FIG. 4, laser tracker 430 sends laser beam 440 to SMR 500.Cube-corner retroreflector 530 reflects the light sent from the lasertracker 430 back to the laser tracker 430 along the same line 440 as theoutgoing laser beam. The laser tracker 430 monitors the position of thereturning laser beam and adjusts the position of the tracker head 450 tokeep the laser beam centered on SMR 500, even as the SMR 500 is movedfrom point to point. In this example, the operator moves the SMR 500 tothree distinct positions (with the portable AACMM 100 held in the samephysical location), but may also move the SMR 500 to twelve or morepositions, or possibly one position only. At each SMR position, both theportable AACMM 100 and the laser tracker 430 take measurements of thecoordinates of the SMR 500. The portable AACMM 100 does this by usingits built-in angular encoders, which typically are located in thecouplings (e.g., bearing cartridges) of the arm segments 106, 108 of theAACMM 100 with each other and with the base 116 of the portable AACMM100 or with the final link 550. Laser tracker 430 does this by using itsdistance meter and angular encoders (not shown). Other types of encodersand distance meters may also be used.

By comparing this SMR measurement data collected by the portable AACMM100 and the laser tracker 430, a transformation matrix may be determinedfor converting from the coordinate system of the portable AACMM 100 tothe coordinate system of the laser tracker 430 or vice versa.Alternatively, both sets of data can be converted into some otherpreferred coordinate system (x, y, z).

When measuring a relatively large object (e.g., an automobile) with aportable AACMM 100, it is often necessary to physically move the entireportable AACMM 100 (or the portion of the AACMM 100 that performs themeasurement function—i.e., the arm portion, with perhaps a laptopcomputer remaining in the same physical location throughout) to adifferent physical location. This is done to physically relocate theportable AACMM 100 to one or more other, different physical locationswith respect to the object being measured so that the portable AACMM 100can measure other portions of the large object that are not accessiblefor measurement from the first physical location of the portable AACMM100. This action of moving the entirety or a portion of the portableAACMM 100 to a different physical location is referred to as“relocation.” That is, “relocation” does not refer to the portable AACMM100 being held in a single physical location while the user moves one ormore portions of the portable AACMM 100 (e.g., arm segments, probe,etc.) to different positions.

The above procedure of measuring the position of the SMR 500simultaneously with both the portable AACMM 100 and the laser tracker430 is performed whenever the portable AACMM 100 is relocated (see FIG.6 where the portable AACMM 100 is moved from “Position A” to “PositionB,” for example—i.e., from a first physical location to a secondphysical location). This allows the measurement data with respect to arelatively large object 600 collected from the several differentlocations of the portable AACMM 100 to be seamlessly stitched togetherin the same common coordinate system in the same frame of reference(e.g., a global frame of reference). With the method described above,the portable AACMM 100 can be quickly and accurately relocated to anyphysical location within the measurement volume of laser tracker 430.

This procedure represents an improvement over other solutions where sucha relatively easy relocation process is not possible because severaltargets (i.e., nests), for example, four targets, are usually placed ona floor as a frame of reference for a portable AACMM 100. Thus, thenests must also be relocated when a portable AACMM 100 is relocated andall points of reference need to be relatively recalibrated.

The following techniques may also be implemented to improve the accuracyof relocating a portable AACMM 100: (1) measure many points (forexample, more than three) with the portable AACMM 100 and the lasertracker 430; (2) measure points separated as much as possible inthree-dimensional space (that is, near the outer edges of the AACMMmeasurement volume or envelope); and (3) measure points covering allthree dimensions (that is, avoid collecting points that lie entirely onor near a plane).

When retroreflector clamp assembly 410 is first attached to the portableAACMM 100, the coordinates of SMR 500 must be found in relation to theframe of reference of the final link 550 of the AACMM 100 (see FIG. 9B).To do this, a compensation procedure is performed using the mountedsphere 700 shown in FIG. 7. This may also be termed an “initialcompensation” procedure, because is it only necessary to be performedwhen the retroreflector clamp assembly 410 is first attached to theportable AACMM 100.

The mounted sphere 700 comprises metal sphere 710, magnetic nest 720,and base 730. Metal sphere 710 may have the same diameter as that of theSMR 500, for example. Magnetic nest 720 has three point-like contacts(not shown) onto which the metal sphere 710 rests. A magnet (not shown)holds metal sphere 710 securely against the three point-like contacts.Magnetic nest 720 is attached to base 730, which in turn is attached tothe floor on another stable surface.

At the start of the compensation procedure to find the SMR position, SMR500 is removed from kinematic nest 510. Kinematic nest 510 is brought incontact with metal sphere 710, which is sitting on magnetic nest 720.This is shown in FIG. 8A. Subsequently, the arm segments 106, 108 of theportable AACMM 100 are moved into a different position, as shown in FIG.8B. The exact position of kinematic nest 510 is not important. Byrepositioning the arm segments at least one more time, but preferablymany more times, the angles on the angular encoders of the portableAACMM 100 can be used to determine the position of the center of SMR500.

The mathematics for determining coordinates using articulated armsegments such as are found on robots or portable AACMMs are well known.For example, the relevant equations are described in chapters 3 and 4 ofRobot Modeling and Kinematics by Rachid Manseur. With these equations,one can relate the position {right arrow over (r)}′ of the center ofmetal sphere 710 within the frame of reference of final link 550 to theposition {right arrow over (r)} of the center of metal sphere 710 withinthe fixed frame of reference of the base 116 of the portable AACMM 100.Possible vectors {right arrow over (r)}′ and {right arrow over (r)} fora particular AACMM are shown in FIGS. 9A and 9B. To clarify the meaningof these vectors, local coordinate systems (x_(A), y_(A), z_(A)) and(x_(F), y_(F), z_(F)) for the portable AACMM 100 and the final link 550,respectively, are shown in FIGS. 9A and 9B. In the case of thecompensation procedure to find the coordinates of the SMR 500 asdescribed above, the constraint of metal sphere 710 causes vectors{right arrow over (r)}′ and {right arrow over (r)} to remain constanteven as the arm segments of the portable AACMM 100 are moved about. Theequation that relates the two vectors is:{right arrow over (r)}=T({right arrow over (θ)}_(i))·{right arrow over(r)}′  (Eq. 1)

In this equation, T({right arrow over (θ)}_(i)) is a 4×4 transformationmatrix that depends on the so-called Denavit-Hartenberg (DH) parametersfor each link, as explained in the book by Manseur cited above. For eachlink, only one of DH parameters, the link angle θ, varies during thecompensation procedure. The other DH parameters are characteristic of aparticular portable AACMM 100 and will already have been determined by afactory compensation procedure carried out at the time the portableAACMM 100 is manufactured. The fixed parameters are determined by aseparate factory compensation procedure. The vector notation {rightarrow over (θ)}_(i) indicates that T is a function of the angularencoder readings for all of the coupling points in the AACMM 100, and iindicates the i^(th) measurement, where each measurement corresponds toa different position of the portable AACMM 100, two example positions ofwhich are shown in FIGS. 8A and 8B. If the portable AACMM 100 is movedto a large number of different positions, there will not be a uniquesolution to Equation (1). Instead, the best estimate of the vectors{right arrow over (r)}′ and {right arrow over (r)} is made by minimizingthe total residual error. For the i^(th) measurement, the residual erroris defined as:a. res_(i) =|T({right arrow over (θ)}_(i))·{right arrow over(r)}′−{right arrow over (r)}|  (Eq. 2)

To minimize the total residual error, {right arrow over (r)}′ and {rightarrow over (r)} are selected to minimize the sum of the square of theres_(i) values. In this case, {right arrow over (r)}′ and {right arrowover (r)} are each represented by three coordinate values (for example,x, y, and z), so that there are six parameter values that need to befound. The procedure for selecting parameters to minimize a sum ofsquared values is well known in the art and is readily carried out usingwidely available software. This procedure will therefore not bediscussed further.

As mentioned previously, the portable AACMM 100 is convenientlyrelocated by simultaneously measuring the position of SMR 500 with boththe AACMM 100 and the laser tracker 430, with SMR 500 moved to severaldifferent positions. The measurements collected by the AACMM 100 arerelated to the measurements of laser tracker 430 through the equation:{right arrow over (s)}=M(rx,ry,rz,tx,ty,tz)·{right arrow over(s)}′  (Eq. 3)

In this equation, {right arrow over (s)} and {right arrow over (s)}′ arethe coordinates of the SMR 500 in the frame of reference of lasertracker 430 and the frame of reference of the portable AACMM 100,respectively. The quantities rx, ry, rz are the Euler anglesrepresenting rotations about the X, Y and Z axes respectively, and tx,ty, tz are the displacements in X, Y and Z respectively. The matrix M(rx, ry, rz, tx, ty, tz) transforms the coordinates of SMR 500, asmeasured by the relocated portable AACMM 100, into the frame ofreference of laser tracker 430 which in this example is the commoncoordinate frame of reference. However, it possible to use, or assign,any suitable frame of reference to be the common coordinate frame ofreference. This matrix M (rx, ry, rz, tx, ty, tz) is the entitydetermined by the relocation procedure, and it the matrix may becomputed in any suitable means such as in a processor or in software,for example. Once it is known, it can equally be applied to ameasurement of a probe tip 420 attached to the final link 550. The probetip 420 coordinate, as measured by the portable AACMM 100, istransformed by matrix M (rx,ry,rz,tx,ty,tz) to give the coordinates ofthe probe tip 420 in the frame of reference of laser tracker 430.

To find M (rx,ry,rz,tx,ty,tz), the residual error for the i^(th)measurement is defined as:res_(i) =|M(rx,ry,rz,tx,ty,tz)·{right arrow over (s)}′−{right arrow over(s)}|  (Eq. 4)

A standard least-squares fit calculation is performed to find the valuesof the six fit parameters rx, Ty, rz, yx, ty, tz that minimize the sumof the squares of the residual errors.

Other apparatus and methods for performing a relocation of the portableAACMM 100 using both the portable AACMM and a laser tracker may beutilized in light of the teachings herein.

In the disclosure herein above, when the portable AACMM 100 is relocatedto a different physical location, the position of the SMR 500 issimultaneously measured by both the portable AACMM 100 and the lasertracker 430. This “simultaneous measurement” is performed each time theSMR 500 is moved to one or more different positions. As such, it isimportant that the portable AACMM 100 and the laser tracker 430 besynchronized in time as accurately as possible such that the any errorin the position of the SMR 500 as measured simultaneously by theportable AACMM 100 and by the laser tracker 430 is as small as possibleand is an acceptable value for such precise metrology applications.

Apparatus and methods according to embodiments of the present inventionmay utilize the IEEE 1588 precision time protocol (PTP) for timesynchronizing both the portable AACMM 100 and the laser tracker 430.Such apparatus and methods may be implemented within both the portableAACMM 100 and the laser 430 tracker of the relocation embodiment ofFIGS. 4-9, discussed herein above. Such apparatus and methods accordingto embodiments of the present invention may utilize the IEEE 1588 PTPfor time synchronizing the real time clocks of many other distributednetwork system combinations of “nodes” (e.g., metrology devices) thatare all similar (e.g., a plurality of like laser trackers) or aredifferent (e.g., a portable AACMM and a laser tracker), or that eveninclude non-metrology devices such as cameras, robots or otherindustrial equipment, for example.

Besides the relocation embodiments of a portable AACMM and a lasertracker and described herein above, it may also be necessary tooccasionally calibrate such a system combination of metrology devices.To perform such a calibration, an SMR 500 may be affixed to the lastlink 550 or to some other location on the portable AACMM 100 (FIG. 4).The laser tracker 430 then measures the position of the SMR 500 relativeto its reference frame while the portable AACMM 100 simultaneouslymeasures the position of its probe 420 (and, thus, that of the SMR 500).From this information, the reference frame of the AACMM 100 relative tothe laser tracker 430 can be established. Then, after calibration, theportable AACMM 100 and the laser tracker 430 may measure independentlyof each other. From time to time the position of the portable AACMM 100may be re-captured (i.e., the SMR 500 re-acquired) to check and confirmcalibration or to relocate the portable AACMM 100 to a new location, asdescribed herein above. As in the relocation embodiments, in thecalibration embodiments of the portable AACMM 100 and the laser tracker430 it is desirable that the portable AACMM 100 and the laser tracker430 be synchronized in time as accurately as possible such that the anyerror in the position of the SMR 500 as measured simultaneously by theportable AACMM 100 and by the laser tracker 430 is as small as possibleand is thus an acceptable value for such precise metrology applications.

As mentioned herein above in the relocation and calibration embodimentsof a portable AACMM 100 and a laser tracker 430 combined together, themeasurement or “acquisition” of the SMR 500 located on the portableAACMM 100 is used to establish the reference location of the portableAACMM 100 so that when the SMR 500 is out of the line of sight of thelaser tracker (e.g., by moving the SMR 500 too quickly or by obstructionof the SMR 500 by an intervening object), measurements made by theportable AACMM 100 may nevertheless proceed uninterrupted. Using aplurality of laser tracker measurements taken for the SMR 500 on theportable AACMM 100 while the AACMM is moving will improve the accuracyin determining the portable AACMM's reference location. There are tworeasons for this. Firstly, from statistics taking N samples reduces theambiguity of each position by the factor

$\frac{1}{\sqrt{N}}.$

Also, using multiple AACMM orientations rather than a single orientationof the portable AACMM 100 reduces the probability that a single worstcase AACMM position error influences the result. Constantly moving theSMR 500 and consequently the encoders within the portable AACMM 100provides the ability to more accurately determine the AACMM referenceposition through use of the average position of the AACMM 100. This isthe same concept used in single point calibration.

With respect to multiple devices (e.g., precision metrology devices suchas portable AACMMs and laser trackers) connected together within adistributed network system (either through wired and/or wirelesscommunications capabilities), to achieve dynamically coordinatedmeasurement and/or control, a communications network typically mustdisplay deterministic behavior. This means always being able to exchangethe required amount of data in a pre-defined time period with theability to precisely time synchronize all participating devices (e.g.,the real time clocks within the portable AACMM 100 and the laser tracker430 in the above disclosed embodiments).

Some of the representative network demands placed on the hardware andsoftware elements include the clock and scheduling, the data managementand message protocols, the bandwidth, the buffering, and thesynchronization. The current wired (IEEE 802.3) and wireless (IEEE802.11) standards provide for the relatively best network solutions.Currently, Ethernet (IEEE 802.3) is the leading candidate for wirednetwork connectivity applications, while Wi-Fi wireless (IEEE 802.11) isthe leading candidate for local wireless network connectivityapplications.

Some of the advantages of Ethernet include it being relativelyinexpensive to implement and offering widespread use, for example, inoffice and industry applications. The Ethernet is also versatile; forexample, wire and fiber configurations may be implemented, as well ashalf or full-duplex implementations. Also, industrial Ethernet protocolsare available (e.g., Real Time Ethernet), and multiple topologies areavailable. Further, Power Over Ethernet (PoE) allows for a single cableto provide signal and power. Ethernet is also relatively easy to secureand segment, and also provides support for the IEEE 1588 PTP, which isdiscussed in more detail herein after.

Some of the advantages of Wi-Fi wireless include it being relativelyinexpensive to implement and offering widespread use, for example, inoffice and industry applications. Also with Wi-Fi wireless stations canbe moved relatively easily. Further, coverage can be extended byrelaying messages. Wi-Fi wireless also offers relatively low powerconsumption; for example, battery power can be used. There is also anabundance of hardware and software for support and Wi-Fi wireless alsoprovides support for the IEEE 1588 PTP.

In the various embodiments of the apparatus and methods of the presentinvention, the relatively high level of accuracy with respect to timesynchronization between the plurality of precision metrology devices isachieved through use of hardware assisted IEEE 1588 time synchronizationoperation, as described in detail hereinafter. For example, anembodiment may comprise a portable AACMM having an LLP connected to theprobe end of the AACMM, along with a laser tracker separate from theAACMM. If the movement of the portable AACMM with the LLP is one meterper second and the synchronization timing error is one millisecond(typical of an IEEE 1588 PTP system using a software only approach), theresulting position error caused by the synchronization timing error isgiven by the product of the arm velocity magnitude and the change in thesynchronization error. In this example, the position error is onemillimeter, which is relatively much larger (two orders of magnitude)than the combined errors of the laser tracker and portable AACMM. Forprecision 3-D metrology, it is desirable that the synchronization timingerror be less than one microsecond (and preferably approximately no morethan 500 nanoseconds) to produce a negligible position error. A hardwareassisted IEEE 1588 PTP approach in general can meet the desired level ofaccuracy whereas a software assisted IEEE 1588 PTP approach in generalcannot.

A clock is typically a two-part device. The first part is an oscillatorthat provides equally spaced (in time) periodic events. The second partis a counter that adds up (also known as an integrator, adder oraccumulator) these events. Perceived time from a clock is influencedprimarily by three elements. The first is caused by delay perturbations(communications path disturbances from the clock to the receiver of thetime information). The second is the variability of the clock time baseitself (type of time base). The third element is any environmentalinfluence over the clock's timing.

To achieve the desired level of time synchronization precision (e.g., amaximum acceptable error of approximately no greater than 500nanoseconds) between a plurality of similar or different precisemetrology devices (e.g., laser trackers, portable AACMMS) and/or othertypes of devices (e.g., industrial robots and equipment) connected,arranged or otherwise combined together, for example, in a system, forwired and/or wireless communications between the devices, it ispreferred that a hardware based implementation of the IEEE 1588 PTP beimplemented in exemplary embodiments of the apparatus and methods of thepresent invention, instead of a software based implementation, forreasons that will be discussed in detail herein after.

As background information regarding the IEEE 1588 PTP, a first version(Version 1) of the standard was released in 2002, while a revisedversion (Version 2) was released in 2008. The IEEE 1588 PTP describes ahierarchical master-slave architecture for clock distribution within anetwork of devices. The IEEE 1588 PTP synchronizes real time clocks indistributed networks. The networked clocks are organized in amaster-slave hierarchy in which a two-way exchange of information occursbetween the master clock device and each of the slave clock devices inorder to keep all of the clocks within the desired precise level of timesynchronization. Under the master-slave hierarchical architecture, atime distribution system comprises one or more communication mediums(network segments) and one or more clocks. An ordinary clock is a clockwith a single network connection and is either the source of (master) ordestination for (slave) a synchronization reference. A boundary clockhas multiple network connections and can accurately bridgesynchronization from one network segment to another. A synchronizationmaster is elected for each of the network segments in the system. Agrandmaster clock is the root timing reference for the entire system.The grandmaster clock transmits synchronization information to the otherclocks residing on the same network segment as the grandmaster. Theboundary clocks with a presence on that segment then relay accurate timeto the other segments to which they are also connected.

In IEEE 1588 Version 2 a type of clock called a transparent clock wasdefined as an improved method of forming cascaded segment topologies. Itis in fact an enhanced Ethernet switch. A transparent clock can replacea boundary clock. A transparent clock monitors PTP messages betweenmaster and slaves and inserts local time delay data (residence time)into the current PTP frame. There are two types of transparent clocks,end-to-end (E2E) and peer-to-peer (P2P). P2P transparent clocks aredifferent than E2E in that each port on a P2P transparent clockadditionally computes the peer link path delay with its link partner (atthe ingress or upstream side) and includes this data in the correctionfield of the PTP frame.

A simplified PTP system frequently comprises a number of ordinary clocksconnected to a single network. In this case no boundary clocks areutilized. A grandmaster clock is elected and all other clockssynchronize directly to it. Generally in the IEEE 1588 PTP a “BestMaster Clock” algorithm determines the “best” or most accurate clockwithin the system to which all other clocks in the system will thensynchronize to. This determination of the master clock is made based ona number of different properties of each clock. For example, clocks maypair up and compare embedded clock attributes. All clocks operate on thesame information and draw the same conclusions.

FIG. 10 illustrates a simplified PTP system in which a grandmaster clock1000 serves as a timebase for the entire system, which includes one ormore slave clocks 1010. IEEE 1588 PTP Version 2 adds some additionalclock types to those within Version 1. In a precision metrology devicesystem, the grandmaster clock 1000 may reside within one of themetrology devices, such as the portable AACMM of FIG. 4, while the slaveclock 1010 may reside within another one of the metrology devices, suchas the laser tracker 430 of FIG. 4. The slave clock 1010 within thelaser tracker 430 may then use the IEEE 1588 PTP to synchronize itselftime-wise to the grandmaster clock 1000 within the portable AACMM 100.This way, measurement accuracy between the two precision metrologydevices 100, 430 may be within an acceptable value of 500 nanoseconds.

Time synchronization and management of a system implementing a hardwareassisted IEEE 1588 PTP is achieved through the exchange of informationin the form of packets or messages (e.g., “PTP packets” or “PTPmessages”) across the communication medium (e.g., the local area network(LAN) or wireless LAN connecting a master and each slave). To this end,the IEEE 1588 PTP uses the following basic messages that are exchangedbetween the various devices: Sync, Delay_Req, Follow_Up and Delay_Resp.These messages are used by the various clocks to communicatetime-related information (“timestamps”) across the network in order totime synchronize each of the slave clocks to a grandmaster clock. Othermessages or packets are available with both versions of the IEEE 1588PTP. Some of the messages are characterized as event messages in thatthey are time-critical messages in which accuracy in transmission andreceipt of timestamp accuracy directly affects clock distributionaccuracy. Other messages are characterized as general messages in thatthey are more general protocol data units in that the data in thesemessages are of import to PTP but their transmission and receipttimestamps are not. The various messages defined within the IEEE 1588PTP may be transmitted using multicasting, which is the type ofdistributed system that Ethernet supports.

FIGS. 11 and 12 illustrate an example of how a slave clock 1010 uses theIEEE 1588 PTP to time synchronize itself to the master clock 1000. Inorder to do so, each slave clock 1010 must accurately determine thenetwork transit time of the Sync messages sent by the master clock 1000.The transit time is determined indirectly by measuring the round-triptime from each slave clock 1010 to its master clock 1000.

At time t0, the master clock 1000 sends the Sync message out to all ofthe slave clocks 1010 via multicasting (in either Version 1 or Version 2of the IEEE 1588 PTP) or to a specific slave clock 1010 via unicastingin Version 2 of the IEEE 1588 PTP. This may be done periodically by themaster clock 1000 (e.g., in Version 1 of the IEEE 1588 PTP it is done upto once per second, while Version 2 allows for the Sync message to bebroadcast by the master clock 1000 up to ten times per second). When aslave clock 1010 receives the Sync message, the slave clock 1010timestamps the local time, t1, of that slave clock 1010. The masterclock 1000 then sends the message Follow_Up to the slave clock 1010. TheFollow_Up message contains the time that the master clock 1000 sent theprior Sync message. Each slave clock 1010 then calculates the timedifference between t1 and t0 to arrive at the offset plus the delay,which, for example is quantity A.

Next, as shown in FIG. 12, the slave clock 1010 sends the Delay_Reqmessage to the master clock 1000 at time t2. The master clock 1000 thensends the Delay_Resp message to the slave clock 1010, wherein theDelay_Resp message contains the time, t3, that the master clock 1000received the Delay_Req message from the slave clock 1010. The slaveclock 1010 then calculates the time difference between t3 and t2 toarrive at the offset minus the delay, which, for example, is quantity B.

The slave clock 1010 can then calculate the offset as (A−B)/2 and canalso calculate the delay as (A+B)/2. Once the slave clock 1010 knows theoffset between itself and its master clock 1000, the slave clock 1010can correct itself by the amount of the offset to bring the slave clock1010 into agreement with the master clock 1000. That is, the time of dayof the slave clock 1010 can be set to the time of day of the masterclock 1000.

Additionally, the IEEE 1588 PTP allows for a slave clock 1010 tocalculate any drift or frequency difference between the oscillator inthe slave clock 1010 and the oscillator in the master clock 1000. Thiscan be accomplished by having the master clock 1000 send out consecutivetimestamped Sync and Follow-Up messages to the slave clock 1010.

With use of the IEEE 1588 PTP, it is known that hardware timestamping ismore accurate in terms of time synchronization than softwaretimestamping. This is achieved primarily through reduction of protocolstack jitter. Jitter typically describes timing deviations of cyclicevents. Network jitter is the jitter caused by the network and itscomponents, including all connected devices. For example, Ethernetsoftware is typically divided up into various layers, where each layerremoves or adds a block of information called a header before handing itup or down the Ethernet protocol stack or “switch fabric.” There are twotypes of models that are documented, which are the OSI (historic) andthe TCP/IP (Internet) model. In each model the top of the protocol stackis that layer which is farthest from the network physical interface,where the layer closest to the network physical interface is generallyreferred to as the PHY layer, which is where a physical device such as anetwork hub resides.

Software timestamping in the IEEE 1588 PTP results in relatively lowertime synchronization accuracy than hardware timestamping because thetimestamping occurs at the top layer of the protocol stack in the masterclock as well as in all slave clocks. As such, the timestamp must traveldown through the entire Ethernet protocol stack in the master clockbefore being placed onto the network. Then the timestamp must travel upthrough the entire protocol stack in each slave clock. As the timestamptravels through the protocol stack, typically many milliseconds ofvariation are introduced due to protocol stack jitter. Also, as thetimestamp travels over the network hundreds of nanoseconds tomicroseconds of variation are introduced by repeater and switch jitter,while milliseconds are typically introduced by router jitter.

In contrast, hardware timestamping is generally carried out closer tothe PHY level in the Ethernet protocol stack. For example, in the masterclock the MAC layer, which is typically located just above the PHY layerin the protocol stack, may carry out the timestamping, while in theslave clock, the PHY layer may carry out the timestamping. In this waythe hardware timestamping technique removes many milliseconds ofvariation introduced by the protocol stack jitter that is inherent insoftware timestamping. Semiconductor integrated circuit devices areavailable (e.g., the model DP83640 from National SemiconductorCorporation) that implement the hardware timestamping at the PHY layerby adding this functionality to the PHY layer in the Ethernet protocolstack.

The IEEE 1588 PTP specifies an algorithm for clock adjustment orcompensation. A widely used algorithm within the IEEE 1588 PTP is toadjust a slave clock based on PTP measurements. This can be accomplishedby speeding up or slowing down the slave clock 1010 (i.e., a rateadjustment) until its offset with respect to the master clock 1000converges to zero (for example, through use of a Proportional/Integral(P/I) servo). The servo input is the “offset” computed from thecommunicated timestamps in the messages. See also the Intel CorporationApplication Note titled “Utilizing IEEE with Intel® EP80579 IntegratedProcess Product Line,” (September 2008), which is incorporated herein byreference in its entirety.

A common known way in the prior art of implementing a variable hardwareclock in a processor within a slave clock 1010 is by use of a phaseaccumulator 1300, as seen in FIG. 13. The phase accumulator is amodulo-M counter of length 2^(n). That is, the value of the 2^(Q)-bitcounter (or accumulator) is incremented by the value M (tuning word)each time a reference clock pulse is received. When the overflow flag(flag when the count exceeds 2^(n)) is used as the output, the followingrelationship between the output frequency and reference frequency isgiven by:F _(out)=(M×F _(ref))/2^(n)  (Eq. 5)where: F_(out) equals the output frequency of the phase accumulator,F_(ref) equals the reference clock frequency (e.g., the processorclock), n equals the length of the phase accumulator, and M equals thetuning word.

Referring to FIG. 14, there illustrated is a block diagram of IEEE 1588PTP clock control using a phase accumulator as is known in the priorart. Some of the definitions used in the following discussion include:FreqOsc is the input clock frequency, FreqClk is the frequencycompensated clock (i.e., the desired output frequency), FreqDivRatio isthe frequency divide ratio which equals FreqOsc/FreqClk, andFreqCompValue is the value used to set the FreqDivRatio. Thecomputational logic elements included within FIG. 14 are a Q-bitAccumulator 1400, an R-bit Addend Register 1404, a P-bit System ClockCounter 1408, an IEEE 1588 Clock Reference 1412, and Message andTimestamp Logic 1416.

The phase accumulator is a Q-bit accumulator to which an addend value isadded at a rate equal to 1/FreqOsc, which will result in a carry rate of1/FreqClk. That is, a carry will result FreqCompValue times in 2^(Q)clock tics, where a clock tic occurs every 1/FreqOsc seconds. If thecarry out is used as the FreqClk (the nominal clock rate desired) thenthe following is derived. Let FreqCompValue equals Addend. For R<=Q

$\begin{matrix}{{FreqClk} = {{FreqOsc} \cdot \left( \frac{FreqCompValue}{2^{Q}} \right)}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

Repeat Pattern: A periodic pattern (beginning and ending withaccumulator equal 0) occurs with a number of carries per cycle equalsFreqCompValue/gcd(FreqCompValue, 2^(Q)) for a number of clock tics(additions) equals 2^(Q)/gcd (FreqCompValue, 2^(Q)). Note the greatestcommon divisor (gcd) of two or more non-zero integers is the largestpositive integer that divides both numbers without remainder.

Frequency and Time Adjustment Resolution: From the definitions above

$\begin{matrix}{{{FreqDivRatio} = {{{FreqOsc}/{FreqClk}} = \frac{2^{Q}}{FreqCompValue}}}{{FreqCompValue} = \frac{2^{Q}}{FreqDivRatio}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

Frequency: For a change in the FreqCompValue the change in FreqClk is asfollows:

$\begin{matrix}{{\Delta\;{FreqClk}} = {\frac{\Delta\;{FreqCompValue}}{2^{Q}} \cdot {FreqOsc}}} & \left( {{Eq}.\mspace{14mu} 8} \right) \\{\frac{\Delta\;{FreqClk}}{FreqClk} = {\frac{\Delta\;{FreqCompValue}}{2^{Q}} \cdot \frac{FreqOsc}{FreqClk}}} & \left( {{Eq}.\mspace{14mu} 9} \right) \\{\frac{\Delta\;{FreqClk}}{FreqClk} = {\frac{\Delta\;{FreqCompValue}}{2^{Q}} \cdot {FreqDivRatio}}} & \left( {{Eq}.\mspace{14mu} 10} \right)\end{matrix}$

Time: The period of FreqOsc and FreqClk are

$T_{FreqOsc} = \frac{1}{FreqOsc}$ $T_{FreqOsc} = \frac{1}{FreqClk}$

A change in FreqCompValue results in a change in the period of FreqClkas follows:

$\begin{matrix}{{\Delta\; T_{FreqClk}} = {{- \Delta}\;{T_{FreqOsc} \cdot \left( \frac{2^{Q}}{FreqCompValue} \right) \cdot \frac{\left( \frac{\Delta\;{FreqCompValue}}{FreqCompValue} \right)}{\left( {1 + \frac{\Delta\;{FreqCompValue}}{FreqCompValue}} \right)}}}} & \left( {{Eq}.\mspace{14mu} 11} \right)\end{matrix}$

Using Equation 7

$\begin{matrix}{{\Delta\; T_{FreqClk}} = {{- \Delta}\;{T_{FreqOsc} \cdot {FreqDivRatio} \cdot \left( \frac{\frac{\Delta\;{FreqCompValue}}{FreqCompValue}}{1 + \frac{\Delta\;{FreqCompValue}}{FreqCompValue}} \right)}}} & \left( {{Eq}.\mspace{14mu} 12} \right)\end{matrix}$

Dividing each side by T_(FreqClk) yields the relative change of FreqClkas a function of ΔFreqCompValue.

$\frac{\Delta\; T_{FreqClk}}{T_{FreqClk}} = {\frac{- T_{FreqOsc}}{T_{FreqOsc}} \cdot {FreqDivRatio} \cdot \frac{\left( \frac{\Delta\;{FreqCompValue}}{FreqCompValue} \right)}{\left( {1 + \frac{\Delta\;{FreqCompValue}}{FreqCompValue}} \right)}}$

Using FreqDivRatio=FreqOsc/FreqClk

$\begin{matrix}{\frac{\Delta\; T_{FreqClk}}{T_{FreqClk}} = {- {1\left\lbrack \frac{\left( \frac{\Delta\;{FreqCompValue}}{FreqCompValue} \right)}{\left( {1 + \frac{\Delta\;{FreqCompValue}}{FreqCompValue}} \right)} \right\rbrack}}} & \left( {{Eq}.\mspace{14mu} 13} \right)\end{matrix}$

Approximation.

$\begin{matrix}{\frac{\Delta\;{FreqClk}}{FreqClk} = {{- 1} \cdot \frac{\Delta\; T_{FreqClk}}{T_{FreqClk}}}} & \left( {{Eq}.\mspace{14mu} 14} \right)\end{matrix}$

Note that the jitter (phase step correction)

$\begin{matrix}{{{Jitter}(ɛ)} = {\pm \frac{1}{FreqOsc}}} & \left( {{Eq}.\mspace{14mu} 15} \right)\end{matrix}$

Specific to the aforementioned DP83640 integrated circuit: FreqOsc=125MHz, Q=35, FreqDivRatio=1.00006399.

Using Equation 8

${\Delta\;{FreqClk}} = {\frac{\Delta\;{FreqCompValue}}{2^{Q}} \cdot {FreqOsc}}$

The change in FreqClk can be expressed as a function of FreqOsc(reference).

$\begin{matrix}{\frac{\Delta\;{FreqClk}}{FreqOsc} = \frac{\Delta\;{FreqCompValue}}{2^{Q}}} & \left( {{Eq}.\mspace{14mu} 16} \right)\end{matrix}$

The resolution of changes in FreqClk relative FreqOsc can be found byletting FreqComp Value=1 (i.e. per increment).

$\frac{\Delta\;{FreqClk}}{FreqOsc} = {\frac{1}{2^{Q}} = {2.9103830457 \times 10^{- 11}{per}\mspace{14mu}\Delta\;{freqCompValue}}}$increment.

Re-arranging Equation 15ΔFreqCompValue=2^(Q)*(ΔFreqClk)/FreqOsc)

Since FreqOsc/FreqClk=FreqDivRatio we can express the required change inΔFreqCompValue for a change of ΔFreqClk/FreqClk by multiplying the leftside by the FreqDivRatio.ΔFreqCompValue=2^(Q)·FreqDivRatio(ΔFreqClk/FreqClk)  (Eq. 17)

EXAMPLE Solve for ΔFreqCompValue to Adjust ΔFreqClk/FreqClk 1953 ppm.

ΔFreqCompValue=2^(Q)·FreqDivRatio·(1953·10⁻⁶)ΔFreqCompValue=67108863(3FF FFFFh DP83640 correction value)

With regard to wireless network types, the network topology of aninfrastructure wireless local area network (WLAN) is a point-to-pointmultipoint star, with a single central controller (e.g., a wirelessaccess point or a wireless router), and one or more wireless clients(e.g., stations). Clients can connect to one and only one access point,and not to other clients. An access point can connect only to clientsand not to other access points. An access point is similar to a basestation in a cellular network, and may perform wireless-to-wiredbridging. An access point may connect with a distribution system orbackbone network, which typically is the Ethernet. In a larger office,or a large and oddly shaped residence, a single access point may notprovide coverage out to the corners of the area. The overall 802.11standard provides for a network in which multiple access points areconnected to the wired portion of the network, operating from the samerouter. Such a network is called an extended service set (ESS). In anESS all access points share the same network name or service setidentifier (SSID). In a basic service set (BSS), a group ofcommunicating stations are arranged in a basic service area, similar toa cell in a cellular network. One may switch between different modes;for example, either an access point or a client, but not both at thesame time.

In contrast to an infrastructure network (WLAN), the network topology ofan ad hoc WLAN is a fully connected matrix, with two or more wirelessclients connecting to each other, without a central controller.Connecting an ad hoc network to other networks (e.g., the Internet)typically involves bridging or routing in one of the wireless clients(e.g., a Windows Internet Connection Sharing) to some other networkconnection.

In embodiments of the apparatus and methods of the present invention, aninfrastructure network may be utilized and may be preferred as such anetwork is relatively easier for security maintenance and development ofthe IEEE 1588 PTP master clock algorithm. However, other embodiments ofthe present invention that utilize an ad hoc network are not precluded.

A wireless access point is the central bridge device used in aninfrastructure (as opposed to an ad hoc) wireless network. Traffic fromthe wireless side of the bridge is sent to the Ethernet or wired side ofthe bridge and vice versa. The access point typically controls allwireless traffic. A wireless router typically comprises three sections:(1) an Ethernet router having an DHCP client and server, a networkaddress translation (NAT) that translates IP addresses, a port addresstranslation (PATP that translates ports, and optionally a firewall; (2)an Ethernet switch; and (3) an access point. Additional elements mayinclude a DSL modem, a cable modem or a wide area network (WAN) port tointerface with either the DSL modem or cable modem. Note that a wirelessrouter, oftentimes less expensive than an access point, can beconfigured to work as an access point.

In exemplary embodiments of the apparatus and methods of the presentinvention, a network configured for the IEEE 1588 PTP may have thefollowing attributes. For example, the domain separation may comprise asingle subnet (i.e., only one master/boundary clock). FIG. 15 is a blockdiagram showing a plurality of metrology devices together with otherdevices connected by wired and wireless connections to a wireless routerand on to a network. For example, a portable articulated arm coordinatemeasuring machine (AACMM) 1500 similar to that of FIG. 1, along with anAACMM having a scanner attached to the AACMM (i.e. arm plus scanner1504) and a laser tracker 1508 may each be connected by a wiredconnection to a wireless router 1512, as indicated by the dashed linesin FIG. 15. Other third party devices 1516 such as sensors, robots, etc.may also be connected by a wired or wireless connection to the wirelessrouter 1512. In addition, a scanner and/or camera station 1520 may notbe directly connected to the router 1512 but may neverthelesscommunicate wirelessly. The wireless router 1512 acts as an access pointfor the LAN that includes the various devices 1500, 1504, 1508, 1512connected thereto by the wired connection, these devices being referredto as “nodes” due to their wired connection to the router 1512, whichthe scanner and/or camera station 1520 being referred to as a “station”due to its wireless connection to other devices with which itcommunicates. The router 1512 also acts as protection for the wireddevices 1500, 1504, 1508, 1516 being in wired connection in an EthernetLAN type of connection with the router 1516. That is, the router 1512protects the Ethernet connected devices from the Enterprise Network orReal Time Industrial Network to which the router is also attached.

In FIG. 15, the devices 1500, 1504, 1508, 1516 may be Wi-Fi stationsthat are implemented as a defended or “protected” network cluster or“cloud.” That is, while these devices are connected to the router 1512,they nevertheless can communicate wirelessly with the access point ofthe router, to exchange measurement data. As such, it is important thatthese devices be accurately synchronized in time using the IEEE 1588 PTPin embodiments of the present invention. Essentially, the devices 1500,1504, 1508, 10516 are taking measurement data in the foreground andexchanging the data with one another via the access point in theforeground, while the embodiments of the apparatus and methods of thepresent invention are time synchronizing the devices 1500, 1504, 1508,1512 in the background.

The resulting IEEE 1588 PTP wireless network group is configured suchthat no “hidden nodes” are allowed. Also, no station data forwarding(routing) is to be supported, and no mobile (come and leave coveragearea) stations are to be supported. Security is to be implemented per802.11i (a preferred method is WPA2).

FIG. 16 is a block diagram illustrating a simplified view of a wirelessrouter having IEEE 1588 PTP master, boundary or optional transparentclock (IEEE 1588 PTP Version 2) support. The WAN Ethernet link 1600contains the actual boundary clock hardware. Also, for each Ethernet LANconnection 1604, the IEEE 1588 PTP timestamp occurs between the PHYlayer 1608 and the MAC layer 1612 within the Ethernet protocol stack1616 or “switch fabric.”

FIG. 17 is a block diagram illustrating the hierarchy of clocks utilizedby the IEEE 1588 PTP in an exemplary embodiment of the present inventionwith various wired nodes 1700 in a LAN domain and various wirelessstations 1704 in a WLAN domain connected by wired and wirelessconnections to a central router 1708. Routers 1708 such as the oneillustrated in FIG. 17 having support for the IEEE 1588 PTP arecommercially available.

Embodiments of the present invention provide for a plurality of similaror dissimilar metrology and/or other, non-metrology devices (e.g.,robots, cameras, industrial equipment, etc.) to be relatively preciselysynchronized in time so that the devices can accurately share databetween them using both wired and wireless communications over adistributed network of the devices. The shared data may, for example, bemeasured data from the metrology devices. The time synchronization ispreferably carried out using the IEEE 1588 PTP. As such, for precisionmetrology devices such as articulated arm coordinate measurementmachines, laser trackers, etc., it is desirable that the network provideless than two microseconds of accuracy and less than 250 nanoseconds ofpeak-to-peak jitter between the router/boundary clock 1708 (FIG. 17) andall slave nodes 1700 and/or wireless stations 1704 for relativelyprecise metrology performance.

What has been described and illustrated so far is known as an “On Path”IEEE 1588 PTP implementation. That is all elements (e.g., router, nodesand stations) support IEEE 1588 PTP time stamping. Basically there is agrand master/boundary clock or master clock, or a network switch withtransparent clock capability. Note that a transparent clock (an enhancedEthernet switch and unique to IEEE 1588 PTP Version 2) measures andadjusts the PTP timestamps to account for packet delay within theswitch. PTP messages are forwarded but modified for the residence timefor the message to propagate from an ingress port to an egress port.Corrections must be made for both Sync_msg and Delay_Req messages. Thedashed IEEE 1588 timestamp elements shown in FIG. 15 support thiscapability.

It is not necessary to implement all the PTP components described hereinabove. In a network comprising all or some standard Ethernet devices(little or no IEEE 1588 support) PTP can accurately be performed byusing only a master and end node (slave). This may be accomplished byincreasing the sync_msg and delay_req rate (up to 124 transactions/secper IEEE 1588 PTP Version 2) and detecting “Lucky Packets” (minimumdelay). A “Lucky Packet” algorithm, which is considered to be primarilya wired network improvement, can be used to improve the timing stabilityfor the wired/wireless connection. This is done by detecting the minimumdelays of “Lucky Packets” and using the results from these packets tomake rate and time corrections. For more information, see NationalSemiconductor Corporation Application Note 1963 “IEEE 1588Synchronization over Standard Networks Using the DP83640”, which isincorporated herein by reference in its entirety. VLAN (virtual LAN—thisis a communication between a group as if they were attached to the samebroadcast domain) techniques can also be helpful. Further, it is notrequired that the boundary clock be resident within the wireless router.WLAN or LAN ports may be used for this access.

Referring to FIG. 18, there illustrated is a block diagram of thefunctions performed within a slave node device (e.g., a metrology devicesuch as those described herein above) implementing the IEEE 1588 PTP.The node is either exclusively an Ethernet node (LAN configuration) or aWLAN station at any one time, but never both at the same time. What thismeans is that packets, including PTP packets, whether from a wired orwireless network are forced to share a common IEEE 1588 PHY device 1800,such as the model DP836450 from National Semiconductor Corporation. Inessence, PTP packets are treated the same without regard to the mediaused (i.e., wired or wireless). That is, a LAN or WLAN connection usesthe same IEEE 1588 PHY device 1800 whether the data is from a wired or awireless connection. As such, the IEEE 1588 PTP packets or messages aretimestamped in the same way regardless of whether it is a wired orwireless connection that the IEEE 1588 PHY device 1800 is dealing with.A LAN switch (multiplexer) 1804 determines which network as between theLAN (e.g., Ethernet) or WLAN (Wi-Fi) is selected. However, as far as thenode is concerned it always appears as an Ethernet LAN. The timestampingalgorithms used are invariant under either setup. For example a “LuckyPacket” algorithm would work the same regardless of media chosen.

Alternatively, the portable AACMM 100 itself may be programmed tocapture data directly from the IEEE 1588 PHY device 1800. For example,an event timer can be set via a PTP clock to facilitate the datacapture. Likewise trigger signals from internal or external sources maybe timestamped as well.

The Ethernet wired LAN signal comes in from an Ethernet connector 1808,while the WLAN wireless connection comes in through an antenna 1812 andthen into a wireless RF module 1816. The output of the IEEE 1588 PHYdevice 1800 is fed to a processing unit 1820. See also FIG. 3A for theIEEE 1588 PTP function 320 within the base processor board 204.

In an embodiment, the user interface 1824 may be a separate processorpartitioned apart from processor 1820. This gives a number ofadvantages, including maintaining legacy configuration; that is, no RTOSis required for the base processor 204. Also, the relatively lowest costcan be obtained by using a base processor 204 implementation only.Further, the user interface and wireless functions can be located remotefrom base processor 204, and relatively simple serial interfaces can beimplemented. In addition, it allows for the relatively best placementconsidering styling and function issues, and allows for modulardevelopment of hardware and software. Further, wireless communicationsis best kept as a separate function, and no real time performance hitoccurs on the base processor 204 due to either or both of the two othermodules. Finally, a TCP/IP stack would be required for the baseprocessor 204.

Referring to FIG. 19, there illustrated is a more detailed block diagramof a hardware time stamp within an IEEE 1588 PTP PHY device.

Referring to FIG. 20, there illustrated is a functional diagram oftiming control and external signal generation within an IEEE 1588 PTPPHY device. The model DP83640 available from National SemiconductorCorporation has an architecture that not only supports timestamping atthe PHY level, but it also provides for a synchronized clock to begenerated for external devices. This external clock nominal frequencycan be any frequency related to 250 MHz divided by n, where n is aninteger in the range of 2 to 255. This clock is generated by one of twoselectable analog oscillators and offset by the rate information in thePTP_RATE registers. Therefore it is frequency accurate to the IEEE 1588PTP clock time. Additionally it has no frequency jumps related to itsreference frequency (125 MHz) as a phase accumulator derived signalwould.

Referring to FIGS. 21 and 22, another feature of embodiments of thepresent invention is support for Synchronous Ethernet (SyncE).Synchronous Ethernet refers to clock recovery (and Phase Lock Loopfiltering) from the receiving node and forward it to the nodestransmitting synchronization. Note that Synchronous Ethernet producesonly frequency synchronization while PTP produces time and frequencysynchronization. Synchronous Ethernet and the IEEE 1588 PTP can beemployed concurrently on a wired Ethernet network (but not on thewireless network as there is no “upstream” clock). When the SynchronousEthernet mode is enabled on PTP slave nodes (and only slave nodes) thetime offset and jitter performance is improved. For more information,refer to National Semiconductor Corporation Application Note 1730,“Synchronous Ethernet Mode: Achieving Sub-nanosecond Accuracy in PTPApplications,” which is incorporated by reference herein in itsentirety.

Referring to FIG. 23, there illustrated is a wireless station that mayinclude an embedded system 2300, a commercially available WIZnetWIZ610wi WLAN Wi-Fi module 2304, and a router 2308. The Wi-Fi module2304 provides a means of bridging wired (Ethernet LAN) and wireless(WLAN) networks. Bridging is a forwarding technique used inpacket-switched networks and no assumptions concerning network addressesare made (as in a router). The Wi-Fi module 2304 provides threeinterfaces: a low speed TTL serial (for device setup and status); an MII(MAC level Ethernet); and a wireless radio. From a system perspective,by connecting a non-IEEE 1588 PHY (i.e., a standard Ethernet PHY) to theMII interface (as in FIG. 18), a transition is made from Ethernet towireless and back to Ethernet.

In the background section above, examples were given of applications inwhich relatively high time synchronization accuracy is needed whenmultiple metrology instruments are used together. One of theseapplications was discussed in detail herein above. The otherapplications are described further below.

Simultaneous multilateration: Simultaneous multilateration is a methodof measuring with relatively high accuracy the position of aretroreflector target by simultaneously measuring the distance to thetarget using at least three, but preferably four, laser trackers. Lasertrackers may contain either or both of two types of distance meters:interferometers and absolute distance meters. Usually the distance tothe retroreflector target is measured using the tracker interferometer,but alternatively the ADM may be used. An example of multilaterationusing tracking interferometers is given in U.S. Pat. No. 4,457,625 toGreenleaf.

Radial distances, which are found by interferometers or absolutedistance meters in laser trackers, are usually at least five times moreaccurate than transverse distances, which are found by the anglemeasuring devices in trackers. For this reason, simultaneousmultilateration, which relies on radial distance measurements, is moreaccurate than tracker measurements based on angle readings.

Synchronization is not important for simultaneous multilateration of astationary retroreflector target. However, for multilateration of amoving target, accurate synchronization is essential. For example,suppose that the target moves at one meter per second andsynchronization is accurate to five milliseconds. The resulting error in3-D position is then on the order of five mm, which is relatively toolarge for this application. If, on the other hand, the synchronizationwere accurate to ten microseconds, the error would be on the order often micrometers, and this error could be further reduced by using Kalmanfiltering or similar methods. Embodiments of the present inventionutilizing the IEEE 1588 PTP would enable this improved synchronization.

Wireless synchronization: As described herein above, it is common toattach a scanner to the end of an AACMM. The scanned data provided bythe scanner must be synchronized to data collected by the angularencoders within the AACMM. Since wires are used to make theseconnections and since the electronics may be controlled by a singlemaster clock, relatively accurate synchronization is possible. In thefuture, however, the very high data rates collected by scanners willlikely make it difficult to transfer the large quantity of data througha small number of wires. In addition, the use of dedicated wires throughthe arm segments limits flexibility in attaching different types ofdevices to the end of the arm. A better method is to use a high speed,yet accurately synchronized, method of wireless data transfer, whichembodiments of the present invention provide.

Compensation and calibration of mid-range scanner: A scanner is a devicethat measures the 3-D coordinates of objects by scanning a source oflight over the surfaces of interest. A mid-range laser scanner is ascanner that measures to a maximum distance of anywhere from a fewmeters to a few hundred meters. Scanners operating over this range havea need for compensation and calibration of their distance sensor. Forexample, the distance measured by such scanners may depend on the powerof light returning to the scanner or it may vary periodically as afunction of distance from the scanner to the target. Such variations indistance can be corrected by means of compensation parameters enteredinto the firmware or software of the scanner. To obtain thesecompensation parameters, a diffuse target having known characteristicsmay be mounted on a carriage that sits on a rail. This carriage may bemoved by hand along the rail or, preferably, automatically moved bymeans of a motor. On the backside of the diffuse target, aretroreflector may be mounted for measurement by a device such as alaser tracker. To make the collection of data as convenient as possible,it should be possible to step the target quickly from point to point or,preferably, in a continuous motion. This is possible if synchronizationbetween the scanner and tracker is highly accurate, for example, toabout ten microseconds. Besides compensation of the scanner, it is alsonecessary to calibrate the laser scanner. Calibration is the procedureby which the performance of the scanner is verified. The same setupdescribed above can be used for calibration as well as compensation,although the conditions should be varied slightly to ensure that thecorrections are being properly applied over all scanner ranges.

Compensation of an AACMM: A two-part method is commonly used tocompensate AACMMs. The first part of the method is to place thespherical probe tip that is attached to the end of the arm into aconical seat or three-point spherical nest. The different segments ofthe arm are then rotated to multiple different positions to fullyexercise the angular encoders within the arm over their full range ofmotion. The second part of the method is to measure one or morecalibrated scale bars. With the information obtained from these twoparts, parameters can be obtained for the arm segment (link) lengths,zero-angle of each encoder, joint offset, and the like. An alternativemethod that may improve arm accuracy is to use the laser tracker tomeasure the position of a retroreflector as the different segments ofthe arm are rotated to multiple different positions. For this method tobe feasible, it is necessary to synchronize the readings of the trackerand arm very precisely.

Some of the advantages of embodiments of the present invention include:(1) the resulting network supports any mix of wired/wireless media(where “wired” media may include not only traditional physical wires orthe like but also optical fibers); (2) all nodes support the IEEE 1588Precision Time Protocol; (3) timing accuracy of less than onemicrosecond between nodes/stations and the router/boundary clock isachievable; (4) industrial networks such as Ethernet/IP, Powerelink,EtherCAT and PROFIINET can be supported; (5) multiple concurrentmeasurement applications can be run; (6) a single host interface issupported; (7) network storage and buffering can be included; (8) asingle “measurement” domain (one boundary clock, unified in the wiredand wireless network) removes accuracy issues when hierarchical boundaryclocks are employed; (9) Synchronous Ethernet (SyncE) mode can beselected in PTP slaves; (10) support for an external grand master clockis supported, including but not limited to GPS; (11) provides multi-bandsolution for high bandwidth applications (scanners); (12) topology canbe extended to other metrology applications; (13) future use of 802.11n(MIMO—multi-antenna) with IEEE 1588 PTP capability can be easilyintegrated.

A 3D robot scanner may be used to inspect heat shields. A solution usingthe IEEE 1588 PTP to synchronize a laser line probe and a commercialrobot could be employed. The software may control the robot and displayand patch 3D scans in real time, thereby providing zooming, measurementof defects and ablator recession rate analysis (surface degradationunder stress).

Embodiments of the method and apparatus of the present invention providefor relatively accurate time synchronization of measurements by multiplemetrology devices such as articulated arm coordinate measuring machines,laser trackers, and other precision metrology devices and evennon-metrology devices such as robots, cameras and industrial equipment.Use of the IEEE 1588 Precision Time Protocol greatly reduces timesynchronization delay by using one master clock for all linked devices,slave clocks for the remaining linked devices, a timestamp routine, andby accounting for delays and offsets. The goal is to achievesynchronization to better than 500 nanoseconds in operation.

Further, to be completely flexible, the synchronization is availableover both wired and wireless media. When this is accomplished, portableAACMMs can be easily mixed and joint 3-D measurements may be made. Thetechnique presented herein is applied to portable metrology equipmentsuch as AACMMs and laser trackers. However, it may be extended to othertypes of metrology devices such as those using Actual Time of Flight(ATOF) or Difference of Time of Arrivals (DTOA) methods.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method, or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablemedium would include the following: an electrical connection having oneor more wires, a portable computer diskette, a hard disk, a randomaccess memory (RAM), a read-only memory (ROM), an erasable programmableread-only memory (EPROM or Flash memory), an optical fiber, a portablecompact disc read-only memory (CD-ROM), an optical storage device, amagnetic storage device, or any suitable combination of the foregoing.In the context of this document, a computer readable storage medium maybe any tangible medium that may contain, or store a program for use byor in connection with an instruction execution system, apparatus, ordevice.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++, C# or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, may be implemented bycomputer program instructions.

These computer program instructions may be provided to a processor of ageneral purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer program instructions may also bestored in a computer readable medium that may direct a computer, otherprogrammable data processing apparatus, or other devices to function ina particular manner, such that the instructions stored in the computerreadable medium produce an article of manufacture including instructionswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the Figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, may be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

While the invention has been described with reference to exampleembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance, but rather the terms first, second, etc. areused to distinguish one element from another. Furthermore, the use ofthe terms a, an, etc. do not denote a limitation of quantity, but ratherdenote the presence of at least one of the referenced item.

What is claimed is:
 1. Apparatus including at least two metrologydevices that communicate with each other in a bidirectional manner, theapparatus comprising: a first one of the at least two metrology deviceshaving an IEEE 1588 precision time protocol interface, the interfaceincluding one or more components configured for bidirectionalcommunications in both a wired manner and a wireless manner with asecond one of the at least two metrology devices; the second one of theat least two metrology devices having an IEEE 1588 precision timeprotocol interface, the interface including one or more componentsconfigured for bidirectional communications in both a wired manner and awireless manner with the first one of the at least two metrologydevices; wherein one of the at least two metrology devices includes amaster clock and the other one of the at least two metrology devicesincludes a slave clock, wherein the master clock communicates a time tothe slave clock and the slave clock is responsive to the communicatedtime from the master clock to adjust a time of the slave clock ifnecessary to substantially correspond to the time of the master clock,thereby time synchronizing the at least two metrology devices together;and wherein each metrology device measures a parameter and wherein themeasurements made by each metrology device occur in a time synchronizedmanner as a result of the time of the slave clock substantiallycorresponding to the time of the master clock.
 2. The apparatus of claim1, wherein the at least two metrology devices are connected in adistributed network.
 3. The apparatus of claim 1, wherein the wiredcommunications occur over an Ethernet local area network.
 4. Theapparatus of claim 1, wherein the wireless communications occur over aWi-Fi network.
 5. The apparatus of claim 1, wherein each metrologydevice comprises a device from the group that includes an articulatedarm coordinate measuring machine (AACMM), an AACMM having a scannerattached thereto, a laser tracker, and a laser scanner.
 6. The apparatusof claim 1, wherein each metrology device comprises a device from thegroup that includes an articulated arm coordinate measuring machine(AACMM) having a scanner attached thereto and a laser tracker, whereineach of the AACMM and the laser tracker track the position of thescanner attached to the AACMM.
 7. The apparatus of claim 6, wherein eachof the AACMM and the laser tracker tracks the position of the scannerattached to the AACMM during one of an AACMM relocation procedure or anAACMM calibration procedure.
 8. The apparatus of claim 2, wherein thedistributed network comprises one from the group that includes a realtime industrial network and an enterprise network.
 9. The apparatus ofclaim 2, wherein the distributed network comprises one from the groupthat includes an infrastructure network and an ad hoc network.
 10. Theapparatus of claim 1, further comprising a router that connects in awired and/or wireless manner with each one of the at least two metrologydevices, wherein the router includes an IEEE 1588 precision timeprotocol interface.
 11. The apparatus of claim 1, wherein the wiredcommunications occur over an Ethernet local area network in aSynchronous Ethernet manner.
 12. The apparatus of claim 1, wherein theIEEE 1588 precision time protocol interface within each of the twometrology devices implements hardware assisted timestamping.
 13. Theapparatus of claim 1, wherein the IEEE 1588 precision time protocolinterface within each of the two metrology devices further comprises aswitch having at a first input thereof a wired connection from the otherone of the at least two metrology devices and having at a second inputthereof a wireless connection from the other one of the at least twometrology devices, wherein the wireless connection from the other one ofthe at least two metrology devices includes one or more components thatconvert the wireless connection to a wired connection that is applied tothe second input of the switch.
 14. The apparatus of claim 1, whereinthe IEEE 1588 precision time protocol interface within each of the twometrology devices implements hardware assisted timestamping such thatwhen the master clock communicates a time to the slave clock and theslave clock is responsive to the communicated time from the master clockto adjust a time of the slave clock if necessary to correspond to thetime of the master clock with a difference between the time of themaster clock and the time of the slave clock being no greater than 500nanoseconds.
 15. A method for communicating between at least twometrology devices in a bidirectional manner, the method comprising thesteps of: providing a first one of the at least two metrology deviceswith an IEEE 1588 precision time protocol interface configured forbidirectional communications in both a wired manner and a wirelessmanner with a second one of the at least two metrology devices;providing the second one of the at least two metrology devices with anIEEE 1588 precision time protocol interface configured for bidirectionalcommunications in both a wired manner and a wireless manner with thefirst one of the at least two metrology devices; wherein one of the atleast two metrology devices includes a master clock and the other one ofthe at least two metrology devices includes a slave clock, the masterclock communicating a time to the slave clock and the slave clock isresponsive to the communicated time from the master clock for adjustinga time of the slave clock if necessary to substantially correspond tothe time of the master clock, thereby time synchronizing the at leasttwo metrology devices together; and wherein each metrology devicemeasures a parameter and wherein the measurements made by each metrologydevice occur in a time synchronized manner as a result of the time ofthe slave clock substantially corresponding to the time of the masterclock.
 16. The method of claim 15, further comprising connecting the atleast two metrology devices in a distributed network.
 17. The method ofclaim 15, wherein the wired communications occur over an Ethernet localarea network.
 18. The method of claim 15, wherein the wirelesscommunications occur over a Wi-Fi network.
 19. The method of claim 15,wherein each metrology device comprises a device from the group thatincludes an articulated arm coordinate measuring machine (AACMM), anAACMM having a scanner attached thereto, a laser tracker, and a laserscanner.
 20. The method of claim 15, wherein each metrology devicecomprises a device from the group that includes an articulated armcoordinate measuring machine (AACMM) having a scanner attached theretoand a laser tracker, further comprising each of the AACMM and the lasertracker tracking the position of the scanner attached to the AACMM. 21.The method of claim 20, further comprising each of the AACMM and thelaser tracker tracking the position of the scanner attached to the AACMMduring one of an AACMM relocation procedure or an AACMM calibrationprocedure.
 22. The method of claim 16, wherein the distributed networkcomprises one from the group that includes a real time industrialnetwork and an enterprise network.
 23. The method of claim 16, whereinthe distributed network comprises one from the group that includes aninfrastructure network and an ad hoc network.
 24. The method of claim15, further comprising a router that connects in a wired and/or wirelessmanner with each one of the at least two metrology devices, wherein therouter includes an IEEE 1588 precision time protocol interface.
 25. Themethod of claim 15, wherein the wired communications occur over anEthernet local area network in a Synchronous Ethernet manner.
 26. Themethod of claim 15, wherein the IEEE 1588 precision time protocolinterface within each of the two metrology devices implements hardwareassisted timestamping.
 27. The method of claim 15, wherein the IEEE 1588precision time protocol interface within each of the two metrologydevices further comprises a switch having at a first input thereof awired connection from the other one of the at least two metrologydevices and having at a second input thereof a wireless connection fromthe other one of the at least two metrology devices, wherein thewireless connection from the other one of the at least two metrologydevices includes one or more components that convert the wirelessconnection to a wired connection that is applied to the second input ofthe switch.
 28. The method of claim 15, wherein the IEEE 1588 precisiontime protocol interface within each of the two metrology devicesimplements hardware assisted timestamping such that when the masterclock communicates a time to the slave clock and the slave clock isresponsive to the communicated time from the master clock to adjust atime of the slave clock if necessary to correspond to the time of themaster clock with a difference between the time of the master clock andthe time of the slave clock being no greater than 500 nanoseconds.
 29. Acomputer program product for implementing communications between atleast two metrology devices in a bidirectional manner, the computerprogram product comprising a non-transitory storage medium havingcomputer readable program code embodied thereon, which when executed bya computer causes the computer to implement a method, the methodincluding the steps of: providing a first one of the at least twometrology devices with an IEEE 1588 precision time protocol interfaceconfigured for bidirectional communications in both a wired manner and awireless manner with a second one of the at least two metrology devices;providing the second one of the at least two metrology devices with anIEEE 1588 precision time protocol interface configured for bidirectionalcommunications in both a wired manner and a wireless manner with thefirst one of the at least two metrology devices; wherein one of the atleast two metrology devices includes a master clock and the other one ofthe at least two metrology devices includes a slave clock, the masterclock communicating a time to the slave clock and the slave clock isresponsive to the communicated time from the master clock for adjustinga time of the slave clock if necessary to substantially correspond tothe time of the master clock, thereby time synchronizing the at leasttwo metrology devices together; and wherein each metrology devicemeasures a parameter and wherein the measurements made by each metrologydevice occur in a time synchronized manner as a result of the time ofthe slave clock substantially corresponding to the time of the masterclock.
 30. The computer program product of claim 29, in which the methodfurther comprises connecting the at least two metrology devices in adistributed network.
 31. The computer program product of claim 29,wherein the wired communications occur over an Ethernet local areanetwork.
 32. The computer program product of claim 29, wherein thewireless communications occur over a Wi-Fi network.
 33. The computerprogram product of claim 29, wherein each metrology device comprises adevice from the group that includes an articulated arm coordinatemeasuring machine (AACMM), an AACMM having a scanner attached thereto, alaser tracker, and a laser scanner.
 34. The computer program product ofclaim 29, wherein each metrology device comprises a device from thegroup that includes an articulated arm coordinate measuring machine(AACMM) having a scanner attached thereto and a laser tracker, furthercomprising each of the AACMM and the laser tracker tracking the positionof the scanner attached to the AACMM.
 35. The computer program productof claim 34, in which the method further comprises each of the AACMM andthe laser tracker tracking the position of the scanner attached to theAACMM during one of an AACMM relocation procedure or an AACMMcalibration procedure.
 36. The computer program product of claim 30,wherein the distributed network comprises one from the group thatincludes a real time industrial network and an enterprise network. 37.The computer program product of claim 30, wherein the distributednetwork comprises one from the group that includes an infrastructurenetwork and an ad hoc network.
 38. The computer program product of claim29, further comprising a router that connects in a wired and/or wirelessmanner with each one of the at least two metrology devices, wherein therouter includes an IEEE 1588 precision time protocol interface.
 39. Thecomputer program product of claim 29, wherein the wired communicationsoccur over an Ethernet local area network in a Synchronous Ethernetmanner.
 40. The computer program product of claim 29, wherein the IEEE1588 precision time protocol interface within each of the two metrologydevices implements hardware assisted timestamping.
 41. The computerprogram product of claim 29, wherein the IEEE 1588 precision timeprotocol interface within each of the two metrology devices furthercomprises a switch having at a first input thereof a wired connectionfrom the other one of the at least two metrology devices and having at asecond input thereof a wireless connection from the other one of the atleast two metrology devices, wherein the wireless connection from theother one of the at least two metrology devices includes one or morecomponents that convert the wireless connection to a wired connectionthat is applied to the second input of the switch.
 42. The computerprogram product of claim 29, wherein the IEEE 1588 precision timeprotocol interface within each of the two metrology devices implementshardware assisted timestamping such that when the master clockcommunicates a time to the slave clock and the slave clock is responsiveto the communicated time from the master clock to adjust a time of theslave clock if necessary to correspond to the time of the master clockwith a difference between the time of the master clock and the time ofthe slave clock being no greater than 500 nanoseconds.